1. Field of the Invention
The invention relates to a resource manager for a satellite telecommunication system and in particular a system in which data is transmitted in the form of packets and is switched by a packet switch on board a satellite. The satellite can be a geosynchronous satellite or a non-geosynchronous satellite. The packets can be asynchronous transfer mode (ATM) cells, but the device can be adapted for any type of packet, of fixed or variable length.
2. Description of the Prior Art
This kind of telecommunication system includes a plurality of terrestrial stations called user stations which communicate with each other via one or more satellites. They compete to use the resources of the satellite(s). This kind of system includes a resource manager for each satellite to manage the resources of the satellite: the bandwidth of each uplink, the bandwidth of each downlink, and the resources of the onboard switch.
The onboard switch distributes data packets arriving on a plurality of uplinks to a plurality of downlinks in accordance with routing data. This kind of telecommunication system includes a system for assigning time and frequency resources to the uplinks (from user stations to the satellite). This is not enough, however: the switch performs statistical multiplexing at each of its outputs. The data packets do not generally have a constant bit rate, and are instead transmitted in the form of bursts. Conflict results if many packets have the same output as their destination at the same time (i.e. must be supported by the same downlink from the satellite to one or more user stations). The conflict is resolved by means of a buffer, and there is generally one buffer per output. The buffer has a limited capacity, however. Buffer congestion leads to the loss of data packets.
To minimize the loss of packets, this kind of system includes a congestion controller which operates on the user station sending the packets to slow down the flow of packets dynamically during transmission. This kind of system further includes a connection admission controller which accepts the setting up of a new connection only if sufficient resources are available at the time in question. The set of these devices constitutes a satellite resource manager. The set of devices can be on the ground or divided between the ground and the satellite.
This kind of resource manager must satisfy the following constraints:                It must optimize the use of uplink and downlink radio resources.        It must guarantee an acceptable loss rate in the onboard switch, especially if it has only a low buffer capacity.        It must limit the complexity of the control elements of the onboard switch.        It must offer maximum flexibility and reconfigurability.        It must be able to support a very broad and constantly changing spectrum of traffic types.        It must be able to offer and guarantee different qualities of service.        It must remain coherent with the principles and standards relating to the ATM layer, the resource management protocol being located in the medium access control (MAC) layer between the ATM layer and the physical layer.        
Using a demand assignment multiple access (DAMA) protocol in the resource manager to optimize the use of the radio resources of a satellite with no onboard switch is known in the art. In conjunction with a connection admission controller (CAC), a DAMA controller assigns the user stations respective frequencies and time slots on an uplink (and the associated downlink) as a function of demand expressed explicitly or implicitly by the stations. Requests from all the stations using a given satellite are sent to the DAMA controller managing the resources of that satellite and are serviced on a “first come, first served” basis. Several DAMA protocols are known in the art that differ in terms of the algorithms used to perform the demand assignment.
For example, the document “Quality-of-service-oriented protocols of resource management in packet switched satellite”, EMS Technologies, 4th Ka band utilization conference, 1998, describes a DAMA type protocol known as the combined free and demand assignment multiple access (CFDAMA) protocol, which manages resources by splitting them into four parts:                A reserved part (constantly assigned) which requires no requests and which is used for all types of traffic at a constant bit rate or that are not able to tolerate the latency time associated with dynamic assignment (constant bit rate (CBR) traffic) or variable bit rate real time (VBRrt) traffic).        A rate-based dynamic capacity (RBDC) part which operates in accordance with a request/assignment principle, requests being expressed in terms of bit rate.        A volume-based dynamic capacity (VBDC) part which operates in accordance with a request/assignment principle, requests being expressed in terms of volume.        A free part which consists of the remaining capacity after the foregoing capacities have been assigned.The latter three mechanisms can be applied to non-real-time traffic, able to tolerate longer delays than the first mechanism.        
In the case of a satellite including a packet switch, it is not sufficient to manage the resources of the uplinks and downlinks, and it is further necessary to adapt the access protocol to perform congestion control, because unless this can be achieved it is necessary to increase the buffer capacity or to tolerate a high rate of loss of packets due to congestion in the buffers of the onboard switch.
A first prior art manager, shown in FIG. 1, controls congestion independently of the resource assignment protocol. The prior art device includes a central unit OBMC1 which combines a DAMA resource assignment controller DAMAC1 and a first subsystem DCC1 of the congestion controller. These controllers are on board the satellite, but they could be grouped with the connection admission controller in a central ground station.
In the device shown in FIG. 1, the user station UES1 includes a DAMA agent DAMAA1 and a second subsystem SCC of the congestion controller.
Knowing that it requires a given bit rate, the subsystem SCC sends the subsystem DCC1 of the congestion controller a request RR indicating the required bit rate. The subsystem DCC1 responds by indicating an authorized bit rate AR or sends a refusal CO if the buffer for the target output is congested. Independently of this, the agent DAMAA1 sends the controller DAMAC1 a request RQ for the assignment of certain resources. The controller DAMAC1 responds with a resource assignment message (burst frequency time plan—BFTP).
The assignment controller DAMAC1 maximizes the uplink load. The congestion controller subsystem DCC1 minimizes congestion of the buffers of the onboard switch (not shown) by limiting the arrival of traffic at the MAC layer of the station UES1 (for example by using conventional flow control). It therefore contributes indirectly to modulating resource assignment requests sent by the agent DAMAA1, but a consequence of the asynchronous operation of the assignment controller DAMAC1 and the congestion controller subsystem DCC1 is that the capacity of the resources assigned by the assignment controller DAMAC1 does not always match that authorized by the congestion controller subsystem DCC1. This solution is therefore somewhat ineffective and requires buffers of high capacity in the onboard switch.
FIG. 2 shows a second prior art manager. In this device, the congestion controller includes a single subsystem DCC2 in the central entity OMBC2 on board the satellite and the user station UES2 no longer includes any subsystem SCC. Requests RQ′ from the assignment agent DAMAA2 are sent to the assignment controller DAMAC2 and to the congestion controller DCC2. The assignment controller DAMAC2 sends assignment messages BFTP to the agent DAMAA2. The congestion controller DCC2 responds to the agent DAMAA2 by indicating an authorized bit rate AR′ or a refusal CO′ if the buffer for the target output is congested. The information from the congestion controller DCC2 is therefore interpreted directly by the agent DAMAA2 in the medium access control layer MAC of the user station UES2. Using this information, the agent DAMAA2 sends assignment requests RQ′ which are modulated to take into account the congestion of each downlink the agent wants to use. This maximizes the use of each uplink, in contrast to the solution previously described.
FIG. 3 shows a third prior art device. In this device, as in the second device, the user station UES3 no longer includes any congestion controller subsystem SCC. Requests RQ″ from the assignment controller DAMAC3 are sent only to the assignment controller DAMAC3 in the central entity OBMC3 on board the satellite. The assignment controller DAMAC3 sends assignment messages BFTP to the agent DAMAA3. The congestion controller DCC3 sends the controller DAMAC3 a message indicating an authorized bit rate AR″ or a refusal CO″ if the buffer for the target output is congested. These messages are interpreted directly by the controller DAMAC3, which takes account of them when allocating resources to the station UES3. The reaction time of this prior art device is shorter than those of the two devices previously described, since the action of the congestion controller DCC3 is not subject to the time-delay introduced by a satellite-Earth-satellite round trip.
The second prior art device can use alternately two types of congestion control algorithm known as “available bit rate” algorithms:                An explicit rate indication for congestion avoidance (ERICA) algorithm controls the bit rate connection by connection, using dedicated packets to transmit requests or indications.        A broadcast rate assignment (BRCA) algorithm, which is a simplified variant of the previous algorithm, and controls congestion for an uplink/downlink pair, instead of connection by connection.        
The ERICA algorithm is applicable only to the first and second prior art devices (FIGS. 1 and 2).
The second and third devices are better than the first device, but nevertheless have various defects:                Great complexity (because the filling of the buffers of the onboard switch must be monitored continuously to detect congestion).        A buffer memory of at least moderate size, because congestion control can only react after detecting the onset of congestion.        The connection by connection processing of the ERICA algorithm generates a high signaling load (dedicated packets), which is added to the signaling load generated by the assignment controller DAMAC.        The uplink/downlink pair processing of the BRCA algorithm rules out equitable sharing of resources, user station by user station, because it does not take account of their specific requirements.        They can be implemented only on board the satellite, because it is necessary to monitor the filling of the buffers and to obtain a shorter reaction time.        
Moreover, in the implementation of the first and second devices (using the ERICA algorithm), only available bit rate (ABR) traffic is subject to congestion control. All other traffic, even bursty non-real-time traffic, is exempted from congestion control, for example variable bit rate, non-real-time (VBR-nRT) ATM traffic, guaranteed frame rate (GFR) ATM traffic, unspecified bit rate (UBR) ATM traffic.
This imposes heavy constraints on the size of the buffers needed in the onboard switch, for example:                a memory of moderate capacity dedicated to VBR-nRT traffic is required for each output to reduce the equivalent bandwidth of a VBR-nRT connection (calculated by the connection admission controller and representative of the bandwidth to be reserved on the downlink in question).        to absorb the uncontrolled arrival of GFR or UBR traffic bursts, a high-capacity memory dedicated to this traffic is required for each output.        
An object of the invention is to propose a manager that does not suffer from the above drawbacks of the prior art managers.